The User Manager should be backed by a special profile, not the guest one.

chrome/browser/profiles/profile_manager.h

Index: chrome/browser/profiles/profile_manager.h
diff --git a/chrome/browser/profiles/profile_manager.h b/chrome/browser/profiles/profile_manager.h
index d71c1d70513191f683834de6a8cf430864b9c9ed..6ada1bdcb8d87b131488bb655d7483aebd9b4645 100644
--- a/chrome/browser/profiles/profile_manager.h
+++ b/chrome/browser/profiles/profile_manager.h
@@ -139,6 +139,9 @@ class ProfileManager : public base::NonThreadSafe,
   // Returns the full path to be used for guest profiles.
   static base::FilePath GetGuestProfilePath();
 
+  // Returns the full path to be used by the user manager profile.
+  static base::FilePath GetUserManagerProfilePath();
+
   // Get the path of the next profile directory and increment the internal
   // count.
   // Lack of side effects: